The present invention relates to a safety circuit arrangement, a monitoring device comprising a plurality of such safety circuit arrangements and a method for the failsafe monitoring of a movement variable of a moving machine part, in particular for the failsafe monitoring of a rotational speed and/or a rotational position.
The invention particularly relates to the failsafe evaluation of a rotational speed sensor for the protection of a danger area on an automatically operating installation, such as a machine tool, a robot, a conveyor belt or an automatically opening and closing door. Danger areas of such machines or installations are frequently protected by protective fences, light barriers and other devices, so that direct access to the danger area is prevented and/or the installation is shut down when the danger area is entered. However, there are cases in which access to a danger area of an installation cannot be entirely prevented, such as when setting up an automated operating sequence on a machine or an automatic door. To minimize the risk of injury to people, it is known practice for movement variables for the relevant drives, such as the rotational speed and/or the torque of a drive or the speed and/or the force of a moving part of an installation, to be limited to a defined maximum value. By way of example, a machine tool can thus be operated at a reduced, limited speed when a protective door is open. In order to ensure the safety of people in these situations, the dangerous movement variables of the operating drive need to be monitored in a failsafe manner.
Failsafe monitoring is usually implemented by monitoring the relevant variables at least twice and comparing the respective results with one another. In this case, the dangerous movement of the machine or installation is permitted only when and for as long as the redundant measurement results for the dangerous movement variable correspond. In addition, failsafe monitoring is achieved by mutually independent evaluation systems which evaluate one or more measured variables.
DE 100 35 783 A1 discloses a monitoring device for monitoring a rotational speed for a synchronous or asynchronous motor, wherein the rotational speed is detected by means of a rotational speed sensor and is compared with the target rotational speed, and additionally drive currents for the electric motors are measured and are compared with target values that are to be expected. A plausibility comparison between the measured rotational speeds and the measured drive currents allows the rotational speed of the relevant synchronous or asynchronous machine to be monitored redundantly.
DE 101 63 010 A1 discloses a circuit for the failsafe monitoring of a speed for an electrical drive, in which two independent systems monitor measured variables for the electrical drive. The first monitoring unit performs the monitoring by using estimated or measured rotational speed values, with the second monitoring unit taking motor current values or reconstructed voltages and ascertaining therefrom a measured variable for determining the speed of the electrical drive. The two independent monitoring units are connected to one another by means of a communication line and perform a plausibility comparison for the monitoring results.
DE 10 2005 045 284 A1 discloses a rotational speed monitoring device in which two independent control units compare the currents from two motor phases with the voltages from the same motor phases. In addition, the total current from all motor phases is also detected and the rotational speed is ascertained from the ripple in the detected signal. This allows the detection of errors in the monitoring systems.
The known methods and apparatuses are all very specifically directed to the available variables and sensors representing the movement, such as the rotational speed of a drive. However, there are a large number of different sensors which are able to deliver very different signals. By way of example, there are incremental rotary encoders which deliver two different square-wave signals, each having a plurality of square-wave pulses. From the number of pulses and the phase of the signals relative to one another, it is possible to determine the rotational speed and the direction of rotation. The levels of the signals are frequently TTL levels in these cases. However, there are also incremental encoders which deliver signals having HTL levels, i.e. the voltage levels are substantially higher than in the case of TTL signals. Furthermore, there are rotary encoders which deliver analog sine and/or cosine signals, such as resolvers. In this case too, the amplitudes can vary depending on the sensor used. Furthermore, there are specific sensor interfaces which use yet other signals, such as Hyperface®. To date, a monitoring device which evaluates the signals from the rotary encoders needs to have been selected and implemented on the basis of the rotary encoder type used, in each case. It would be desirable to have a monitoring device which can be inexpensively combined with a plurality of different sensor types and which is also compact.